1. Field of the Invention
The present invention relates to an image display apparatus that is used in front of an eye, such as those used as viewfinders in various cameras and as head-mounted displays.
2. Description of the Prior Art
Many video cameras and digital cameras are equipped with an image display apparatus as a viewfinder. On the other hand, image display apparatus of the type called head-mounted displays (HMDs) have been becoming increasingly popular in recent years. These image display apparatus are used in front of an eye, and are typically composed of a display device for displaying an image, a light source for feeding illumination light to the display device, and an eyepiece optical system for directing image light (the light carrying the image) from the display device to the eye so as to present a virtual image of the image displayed on the display device.
Among many types of display devices, reflective liquid crystal panels are in wide use because they are compact but nevertheless offer high resolution and in addition high light use efficiency. However, a reflective liquid crystal panel needs to be illuminated from in front of its display surface, and therefore, unless a complicated arrangement is adopted in which the path of image light deviates greatly from that of illumination light, it is inevitable to dispose in front of the panel a combiner for combining the paths of image and illumination light.
Moreover, a reflective liquid crystal panel reflects illumination light substantially regularly, and therefore, to make the panel uniformly bright with respect to the observer's eye, it is necessary to dispose on the path of illumination light a condenser lens so that the light source, or the secondary light source such as a diffuser panel that is illuminated by the light source, is substantially conjugate with the observation point at which the observer's eye is located.
U.S. Pat. No. 6,023,253 discloses an arrangement in which, between a reflective liquid crystal panel and an eyepiece lens, a polarization separation (PBS) block is disposed as a combiner and, between the combiner and a light source, a condenser lens is disposed. However, in this arrangement, the beam diameter of the illumination light incident on the PBS block is so large that it is impossible to make the block satisfactorily slim. Moreover, the condenser lens is located to the side of the eyepiece optical system, and this makes the apparatus as a whole unduly large.
Japanese Patent Applications Laid-Open Nos. 2000-81519 and 2000-147422 disclose arrangements that permit the omission of the condenser lens between the combiner and the light source. FIG. 18 schematically shows those arrangements. In FIG. 18, reference numeral 51 represents a reflective liquid crystal panel, reference numeral 52 represents an eyepiece optical system, reference numeral 53 represents a light source, and reference numeral 54 represents a combiner. The eyepiece optical system 52 includes positive lenses 52a and 52b, and the combiner 54 is disposed between these two positive lenses 52a and 52b. That is, the positive lens 52b is located between the panel 51 and the combiner 54, and this positive lens 52b functions as a condenser lens as well. This makes it possible to place the light source 53 nearer to the combiner 54 to some degree and thereby make the apparatus as a whole more compact.
However, in the arrangements disclosed in Japanese Patent Applications Laid-Open Nos. 2000-81519 and 2000-147422 mentioned above, the eyepiece optical system has a positive refractive optical element in its portion located on the observation point side of the combiner, and this positive refractive optical element, together with the panel-side positive lens, contributes to a large Petzval sum. As is well known, a large Petzval sum causes large, negative curvature of field. This makes it impossible to present the observer with a high-quality image (virtual image).
Moreover, in the arrangements disclosed in the aforementioned laid-open applications, the observation point is made conjugate with the light source by designing the eyepiece optical system to be substantially telecentric with respect to the liquid crystal panel and by making the illumination light substantially parallel with respect to the liquid crystal panel. Thus, the observation point is located at the front focal point of the eyepiece optical system as a whole, and the light source is located at a point equivalent to the front focal point of that portion of the eyepiece optical system which is located between the combiner and the panel.
As a result, it is extremely difficult to shorten the focal length of the eyepiece optical system and thereby widen the field of view of the image presented while securing a sufficiently long eye relief (the distance from the front end of the eyepiece optical system to the observation point). Moreover, attempting to make the apparatus as a whole still more compact by placing the light source nearer to the combiner leads to an increase in the power of the positive lens disposed between the panel and the combiner and thus to a further increase in the Petzval sum. This makes further miniaturization also difficult.